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Understanding your windows

Window condensation: where is the moisture?

Room-side moisture, exterior dew and fog between sealed panes call for different questions.

Glass Rescue · September 26, 2026

Small condensation droplets on the indoor surface of window glass
Surface condensation detail showing moisture on the room side of the glass.

“My window is foggy” can describe several different things. Start by identifying where the moisture appears before deciding that the glass needs replacement.

Locate the moisture

On the room-facing surface
Indoor humidity and cold glass can produce condensation. Closed blinds or furniture near the window can restrict air circulation. This alone does not establish a failed glass seal.
On the outdoor surface
Exterior dew can appear after a cool, calm, humid night. Outdoor moisture is different from moisture trapped inside a sealed unit.
Between the panes of a sealed unit
Moisture inside a sealed glass unit can indicate seal failure. The insulating glass unit may need replacement; assess the surrounding window before choosing the scope.

Source: Natural Resources Canada: condensation.

Older storm windows are a different arrangement.

Moisture between separate storm windows or unsealed panes is not the same as moisture inside a sealed insulating glass unit. Tell your installer which type you have, or send a wider photo if you are unsure.

NRCan: upgrading windows and exterior doors

What to record

A brief observation is more useful than guessing the cause. Note the room, time of day, weather and whether the moisture clears later. Describe whether it affects one pane, several windows or one side of the house.

If intact glass is safely within reach indoors, note whether a small patch wipes clear. Do not lean outside or climb to check the exterior. If you cannot identify the surface, describe that uncertainty when you ask for an assessment.

  • Take a wider photo showing the whole frame.
  • Add a close-up that captures the moisture.
  • Mention staining, water pooling or problems opening and locking the window.

Choose your next conversation

For room-side moisture, discuss humidity and ventilation as well as the window. For suspected moisture inside a sealed unit, ask about a glass assessment. Recurring water around the frame deserves a closer inspection rather than a conclusion based on a photograph.

Before approving replacement, ask what evidence supports the diagnosis, which parts will be replaced and which will stay. Keep any manufacturer paperwork available so the existing warranty can be checked.

Explore foggy glass replacement or use our photo checklist to prepare your enquiry.

A better description

Trade “it is foggy” for a useful observation.

01

Record the pattern

“The bedroom glass is wet on the room side in the morning” is different from “one pane stays cloudy between the sheets all day.” Note what you can see without reaching outside.

02

Keep the room in the picture

Mention blinds, nearby furniture and whether other rooms are affected. Natural Resources Canada notes that restricted air movement near a window can contribute to surface condensation.

03

Ask a question before buying a fix

“What supports the diagnosis, and what will this work address?” is a useful question. The objective is to solve the observed problem, not merely replace something.

Further reading: Natural Resources Canada — condensation.

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